Eric Holder: No confirmation Americans among Nairobi attackers

9/25/13 1:02 PM EDT

Attorney General Eric Holder said Wednesday that the U.S. lacks confirmation that Americans were among the perpetrators of the recent terrorist attack on a Nairobi shopping mall.

"At this point, we don't have any verification. I've seen those reports," Holder said in response to a reporter's question about whether U.S. citizens took part in the attack.

"We don't have any verification that any Americans have been involved with the attack as attackers. So we are still working with our Kenyan colleagues to get granularity, fidelity as to what the truth is with regard to that matter," the attorney general added. He spoked during a Justice Department news conference about an unrelated case about maniuplation of an index of interbank exchange rates.

At least 67 civilians and security personnel are believed to have died in the attack that began Saturday, although the militant group Al-Shabab claims the death toll climbed much higher after authorities demolished a portion of the malls. Five attackers were also killed, the Kenyan government said.

U.S. officials have been concerned for years that American citizens were traveling to Somalia to join Al-Shabab. Several people in the U.S. have been charged with supporting Al-Shabab financially or through plans to fight for the group. However, there have been no charges against Americans in connection with specific terrorist attacks, such as the one at Nairobi's Westgate mall.